Field of the Invention
The present invention relates to a method for manufacturing lithium carboxymethyl cellulose.
Introduction
Lithium carboxymethyl cellulose (Li-CMC) is a possible binder material for use in lithium ion (Li-ion) batteries. Traditional binder systems typically have used polyvinylidene fluoride (PVDF) as a polymeric binder and N-methyl-2-pyrrolidone (NMP) as a solvent for the binder. The fluorinated binder and hazardous solvent prove challenging to safely handle and dispose. Li-CMC is an alternative binder that can be delivered using an aqueous solvent. As a result, use of a Li-CMC binder can reduce the hazards associated with the binder system of Li-ion batteries and disposal concerns associated with both the binder and the solvent. However, manufacturing Li-CMC can be challenging.
Carboxymethyl cellulose (CMC) is commonly available as a sodium salt (Na-CMC) due to specific manufacturing conditions that include alkalization of cellulosic raw material with caustic soda followed by etherification and neutralization. Therefore, essentially all commercially available CMC is Na-CMC.
CN102206286A discloses a method for converting Na-CMC to Li-CMC using hydrochloric acid. The reference discloses treating Na-CMC with an aqueous hydrochloric acid solution and then treating the resulting acid form of CMC (H-CMC) with an aqueous lithium hydroxide solution to achieve Li-CMC. Unfortunately, treating Na-CMC with hydrochloric acid (a strong acid) generally degrades the CMC polymer and risks corrosion of processing equipment. Additionally, the resulting Li-CMC requires a drying step that can cause crosslinking by re-esterification of the carboxylate groups.
Other methods for converting Na-CMC to Li-CMC include treating with a concentrated aqueous lithium hydroxide solution followed by etherification with chlorine acetic acid. (see, e.g., Machado, G. D. et al., Polimery, 48, 4 (2003) 273-279; and Abuh-Lebdeh et al., Journal of Power Sources, 196 (2011) 2128-2134). However, lithium hydroxide has insufficient strength to fully solubilize the cellulose chain for the subsequent etherification step. It is also known to prepare Li-CMC using an ion exchange column. (See, Abuh-Lebdeh et al, Journal of Power Sources, 213 (2012) 249-254). However, an ion exchange column process is a low volume process that can only produce small quantities of Li-CMC.
There is a need for a less challenging method for manufacturing Li-CMC that does not suffer from the handicaps of the prior art.